According to wireless LAN standards Institute of Electrical and Electronics Engineers (IEEE) 802.11, a task group TGax is studying a next-generation communication method 802.11ax (HEW: High Efficiency Wireless LAN (WLAN)). With regard to component technologies, the next-generation communication method 802.11ax is expected to adopt a new modulation/demodulation method (1024 Quadrature Amplitude Modulation (QAM)), support uplink Multi User Multi-Input Multi-Output (MU-MIMO) and introduce Orthogonal Frequency-Division Multiple Access (OFDMA).
FIG. 1 is a view illustrating a basic configuration example of a wireless LAN system. This wireless LAN system includes an access point AP, and a plurality of terminals STA 1 to STA n. In a case where which one of the terminals STA 1 to STA n is not specified, the terminal will be referred to as a terminal STA below. In addition, FIG. 1 assumes that there are the three or more terminals STAs, yet is not limited to this. When single user communication is performed, the number of the terminal STA may be one (i.e., n is one), and when multi-user communication is performed, the number of the terminals STAs may be two or more (i.e., n is two or more). The access point AP and the terminals STA 1 to STA n form a network called a Basic Service Set (BSS).
In a case of uplink OFDMA introduced in IEEE 802.11ax (HEW) or UpLink Multi-User (UL MU) communication of uplink MU-MIMO, the access point AP performs centralized control of a plurality of terminals STAs, divides communication resources into a plurality of Resource Units (RUs), and allocates the resource units to a plurality of terminals STAs. Consequently, a plurality of terminals STAs can simultaneously perform communication.
A procedure of uplink multi-user communication will be briefly described (see Non-Patent Literature 1). When there is uplink data which waits to be transmitted, the terminal STA notifies in advance the access point AP of a buffer status indicating that there is the uplink data. The access point AP transmits a trigger frame for allocating resource units RUs, to a plurality of terminals STAs in the BSS. In this case, the access point AP includes, in the trigger frame, information of the resource units RUs and terminal Identifier(s) (ID(s)) (e.g., Association ID(s) (AID(s))) for designating the terminal(s) STA(s) to which the resource units RUs are allocated. In addition, the AID is an ID which is uniquely generated by the access point AP to manage the terminal STA associated with the access point AP and is unique to the terminal STA.
However, even when the unassociated terminal STA and the terminal STA which has already been associated yet has woken up from sleep immediately before, have uplink data which waits to be transmitted, the access point AP cannot learn the presence of the uplink data. Therefore, the access point AP cannot allocate the resource units RUs to these terminals STAs. Hence, UL OFDMA-based random access is introduced, and these terminals STAs can perform uplink data transmission (see clause 4.5 of Non-Patent Literature 2).
A procedure of OFDMA-based random access will be described with reference to FIG. 2. In this regard, at least the terminals STA 1 and STA 2 exist in the BSS.
Procedure 1:
The terminal STA which has uplink data which waits to be transmitted sets an initial value to an OFDMA Back-Off (OBO) counter of the terminal STA. A value within a range from 0 to an OFDM Contention Window (OCW) is randomly selected for the initial value.
Procedure 2:
The access point AP transmits a Trigger Frame for Random access (TF-R) for allocating the resource units RUs, to a plurality of terminals STAs in the BSS (time t1). In this regard, the access point AP notifies five resource units RU 1 to RU 5. Further, the access point AP does not allocate the resource units RU 1 to RU 5 to the specific terminals STAs, and sets the resource units RU 1 to RU 5 as the resource units RUs for random access. Hence, the access point AP includes, in the trigger frame for random access, information of the resource units RU 1 to RU 5, a special terminal ID (an AID 0 in an example in FIG. 2) indicating that random access can be made to each of the resource units RU 1 to RU 5. The terminal STA whose count value of the OBO counter is not 0 decrements the count value per predetermined time by five which is the number of the resource units RU 1 to RU 5 for which the special terminal ID has been designated. As a result, the terminal STA whose count value has become 0 selects one of the resource units RUs, and uses the selected resource unit RU to transmit a UL MU Physical Layer Convergence Protocol (PLCP) Protocol Data Unit (PPDU) frame including uplink data to the access point AP (time t2). In this regard, the terminal STA 2 transmits uplink data by using the resource unit RU 1, and the terminal STA 1 transmits uplink data by using the resource unit RU 3.
Procedure 3:
The access point AP transmits ACKnowledgement (ACK) frame(s) for notifying whether or not communication of the UL MU PPDU frame at the time t2 succeeds, to a plurality of terminals STAs in the BSS (time t3). In a case where the terminal STA has not been able to correctly transmit uplink data due to collusion, the terminal STA can decide that the uplink data has not been able to be transmitted by checking the ACK frame. Hence, the terminal STA which has not been able to correctly transmit the uplink data returns to the procedure 1, sets a random initial value within the range from 0 to the OCW to the OBO counter again, and waits for an occasion to retransmit the uplink data.
However, the unassociated terminal STA does not have the AID, and therefore the access point AP cannot include the AID in the ACK frame to notify the unassociated terminal STA of whether or not communication succeeds. Furthermore, when the unassociated terminal STA can join only random access, a state of low communication efficiency continues until the terminal STA is associated. Hence, it is considered to designate the unassociated terminal STA by a temporary terminal ID (e.g., an Unassociation ID (UAID)).
FIG. 3 is a view for explaining an example of the temporary UAID for designating the unassociated terminal STA. In this case, there are at least the associated terminals STA 1 to STA 3, and unassociated terminals STA 4 and STA 5 in the BSS, and an AID 1 to an AID 3 are set as terminal IDs to the associated terminals STA 1 to STA 3, respectively.
At the time t1, the access point AP transmits the trigger frame for notifying the resource units RUs, to a plurality of terminals STAs in the BSS. In this regard, the access point AP notifies the five resource units RU 1 to RU 5. Further, the access point AP allocates the resource units RU 1 to RU 3 to the associated terminals STA 1 to STA 3, respectively, yet does not allocate the resource units RU 4 and RU 5 to the specific terminals STAs and sets the resource units RU 4 and RU 5 as the resource units RUs for random access. Hence, the access point AP includes, in the trigger frame at the time t1, information of the resource units RU 1 to RU 5, the AID 1 to the AID 3 for designating the terminals STA 1 to STA 3 to which the resource units RU 1 to RU 3 are allocated, and an AID 0 which is a special value indicating that the resource units RU 4 and RU 5 are the resource units RUs for random access.
At the time t2, the terminals STA 1 to STA 3 designated by the AID 1 to the AID 3 transmit the UL MU PPDU frames including uplink data to the access point AP by using the resource units RU 1 to RU 3, respectively, allocated to the terminals STA 1 to STA 3. Further, the terminal STA 5 accesses the resource unit RU 4 for random access, obtains an access right to the resource unit RU 4, and transmits the UL MU PPDU frame including uplink data to the access point AP by using the resource unit RU 4. On the other hand, regarding the resource unit RU 5, since collision of the uplink data occurs, successful communication cannot be performed. Thus, the two resource units RU 4 and RU 5 are likely to cause collision of the uplink data in the UL MU PPDU frame at the time t2, and therefore communication efficiency is low.
At the time t3, the access point AP transmits the ACK frames for notifying whether or not communication of the UL MU PPDU frames at the time t2 succeeds, to a plurality of terminals STAs in the BSS.
At the time t4, the access point AP transmits the trigger frame for notifying the resource units RUs, to a plurality of terminals STAs in the BSS. In this regard, the access point AP notifies the five resource units RU 1 to RU 5. Further, the access point AP allocates the resource units RU 1 to RU 3 to the associated terminals STA 1 to STA 3, respectively, and does not allocate the resource unit RU 5 to the specific terminal STA and sets the resource unit RU 5 as the resource unit RU for random access. Further, the access point AP allocates the resource unit RU 4 to the unassociated terminal STA 5, and designates the unassociated terminal STA 5 by a temporary UAID 5. The UAID 5 is designed such that values independently generated by the access point AP and the terminal STA 5 match. For example, the access point AP and the terminal STA 5 process a Media Access Control (MAC) address of the terminal STA 5 by a predetermined process, and generate the UAID 5 such that the UAID 5 fits in an information element (12 bits) for storing the AID in the trigger frame. In this regard, the access point AP and the terminal STA 5 need to generate the UAID 5 such that the UAID 5 can be distinguished from a normal AID value. When the terminal STA 5 is associated and the AID value is set to the terminal STA 5, the access point AP and the terminal STA 5 delete the UAID 5, and finish managing the UAID 5.
As described above, the access point AP includes, in the trigger frame at the time t4, the information of the resource units RU 1 to RU 5, the AID 1 to the AID 3 for designating the terminals STA 1 to STA 3 to which the resource units RU 1 to RU 3 are allocated, the temporary UAID 5 for designating the terminal STA 5 to which the resource unit RU 4 is allocated, and the AID 0 which is a special value indicating that the resource unit RU 5 is the resource unit RU for random access.
At the time t5, the terminals STA 1 to STA 3 designated by the AID 1 to the AID 3 transmit the UL MU PPDU frames including the uplink data to the access point AP by using the resource units RU 1 to RU 3, respectively, allocated to the terminals STA 1 to STA 3. Further, the terminal STA 5 designated by the UAID 5 transmits the UL MU PPDU frame including the uplink data to the access point AP by using the resource unit RU 4 allocated to the terminal STA 5. Further, the terminal STA 4 accesses the resource unit RU 5, obtains the access right to the resource unit RU 5, and transmits the UL MU PPDU frame including the uplink data to the access point AP by using the resource unit RU 5. Thus, only the resource unit RU 5 is likely to collision of the uplink data in the UL MU PPDU frame at the time t5, so that communication efficiency improves.
At the time t6, the access point AP transmits ACK frames for notifying whether or not communication of the UL MU PPDU frames at the time t5 succeeds, to a plurality of terminals STAs in the BSS.